Imagers are calibrated in order to produce an image of higher quality than if not calibrated. One level that is calibrated is the black level of a graphic imager. The black level of an imager has a fixed term, which is systematic offset, and a moving term, which is temporal offset due to changing supply voltages, temperatures, and the like. If these offsets are calibrated out purely in the digital domain then the dynamic range of the system is reduced.
If the offset is half of the Analog-to-Digital Converter's (ADC) range then the signal dynamic range is reduced by half due to this offset. When the offset places the wanted signal below the conversion range of the ADC, then the ADC cannot correctly represent this, and the signal is clipped to a zero code instead of a corresponding negative code. This clipping increases the dead zone of the imager and therefore reduces image quality. Analog domain offset calibration is widely used due to these reasons. If the target black level for calibration is set to be a small positive level, a negative offset can be calibrated without the clipping problem.
Fixed systematic offset can easily be calibrated out through the use of a simple Digital-to-Analog Converter (DAC). The DAC is configured to subtract the systematic offset. A digital circuit evaluates the offset and applies a suitable code to the DAC to remove the fixed systematic offset.
A moving offset, however, is harder to calibrate in this manner. Since the precise effect of the DAC on the signal is not known by a digital circuit, the calibration loop must be closed and iterative. This yields the possibility of oscillations. These oscillations can potentially add interframe noise or flicker noise, whereby the whole luminance level appears to increase/decrease on a frame by frame basis. This flicker noise deteriorates imager performance. What is needed is a black level offset calibration method to yield a high quality image sensor.